If you’re like most people, you down your pills with juice or food, thinking you’re doing your stomach a favor by not taking the medications alone. As it turns out, you may be doing more harm than good due to food-drug interactions.

Recent research shows that foods and beverages can have a dramatic effect on how the body absorbs certain medications.

“Twenty years ago, a team of researchers and I realized that grapefruit juice could cause many medications to be as much as 10 times more powerful than they’re meant to be,” says David G. Bailey, PhD, clinical professor of pharmacology at the University of Western Ontario in Canada.

Thanks to their research, more than 50 medications now carry cautions about grapefruit juice on their labels.

Now Bailey has discovered that grapefruit, apple and orange juices can actually have the opposite effect on some other drugs. “They block the pills’ absorption, so you get less or even no benefit from taking them,” says Bailey. “This might cause someone to think that they need to switch to a ‘better’ medication Р when in fact changing what they eat, or when, would make the drug effective.”

Blocks: Antidepressants, especially monoamine oxidase inhibitors (MAOIs) like tranylcypromine (Parnate) Р although people taking SSRIs like fluoxetine (Prozac) should avoid drinking more than moderate amounts of alcohol as well, as it counteracts their benefits.

How long should you wait?

In most cases, you can consume a potentially pill-blocking food or beverage about three to four hours after you’ve taken your meds, but it’s best to ask your pharmacist, who can tell you for sure. Plus, she can check to see if other medications or supplements you take might further affect how your body absorbs your medicines.